Light: can you feel the force?

A nanosensor in an evanescent field of circularly polarised light experiences a force perpendicularly to the direction of propagation
Dr Massimo Antognozzi

25 April 2016

A ground-breaking technique capable of measuring the extremely weak forces exerted by light – forces smaller than one piconewton – has been developed by scientists at the University of Bristol.

It is well established that light exerts a momentum in the direction of propagation, solar sail propulsion on a spacecraft being just one example of this effect. Surprisingly, light can also carry linear momentum perpendicular to its propagation direction. This component originates from a field-theory construction introduced by Dutch physicist Frederik Jozef Belinfante more than 70 years ago, and it has remained unobserved until now.

The Bristol team, led by Dr Massimo Antognozzi of the School of Physics, used unique nano-sensors to confirm the existence of this extraordinary momentum of light and a new toolkit to study light-matter interaction with unprecedented sensitivity.

The research, reported in the journal Nature Physics, was an international collaboration between research groups in the UK, Japan, the Czech Republic, and Ukraine, and is predicted to have fundamental implications in the way we describe light and the momentum it carries when interacting with matter.

The research was made possible through a generous contribution from Bristol alumni Nick and Susan Woollacott.

Paper: 'Direct measurements of the extraordinary optical momentum and transverse spin-dependent force using a nano-cantilever' by M. Antognozzi et al. in Nature Physics.